Significant loss of soil inorganic carbon at the continental scale.

Widespread soil acidification due to atmospheric acid deposition and agricultural fertilization may greatly accelerate soil carbonate dissolution and CO2 release. However, to date, few studies have addressed these processes. Here, we use meta-analysis and nationwide-survey datasets to investigate changes in soil inorganic carbon (SIC) stocks in China. We observe an overall decrease in SIC stocks in topsoil (0-30 cm) (11.33 g C m-2 yr-1) from the 1980s to the 2010s. Total SIC stocks have decreased by ∼8.99 ± 2.24% (1.37 ± 0.37 Pg C). The average SIC losses across China (0.046 Pg C yr-1) and in cropland (0.016 Pg C yr-1) account for ∼17.6%-24.0% of the terrestrial C sink and 57.1% of the soil organic carbon sink in cropland, respectively. Nitrogen deposition and climate change have profound influences on SIC cycling. We estimate that ∼19.12%-19.47% of SIC stocks will be further lost by 2100. The consumption of SIC may offset a large portion of global efforts aimed at ecosystem carbon sequestration, which emphasizes the importance of achieving a better understanding of the indirect coupling mechanisms of nitrogen and carbon cycling and of effective countermeasures to minimize SIC loss.

[Interactions of Fungi Community and Relationship with the Carbon Structure in Arable Mollisols with Gradient Organic Matter Content].

Fungi play an important role in the accumulation and transformation of soil organic matter (SOM) and nutrient cycling. To investigate the relationship between the fungal community and soil organic carbon functional groups under gradient SOM contents in arable mollisols, arable mollisols with 2%-9% SOM content were collected in Northeast China. Solid-state 13C-NMR technology was used to explore the differences in the functional group structure of SOM, and ITS high-throughput sequencing was used to investigate the fungal community structure. The potential interactions between different taxonomic groups of soil fungal community and their associations with organic carbon molecular structures were compared by constructing molecular ecological networks under low SOM (2%-5%) and high SOM (7%-9%) conditions. The 13C-NMR results indicated an increase in the relative abundance of Alkyl C (25.8% to 35.9%). The decrease in Alkyl C/O-Alkyl C indicated a smaller degree of decomposition in high SOM soils. Sordariomycetes and Mortierellomycotina dominated the fungal community and their relative abundance increased with the SOM gradient (P<0.05) from 14.33% to 28.17% and from 7.32% to 23.14%, respectively. The network analysis showed simpler ecological topological properties of the fungal community in low SOM soils, with lower numbers of nodes, edges, and average clustering coefficients than those in high SOM soils. A closer relationship between fungi and organic carbon functional groups, especially LOC, was observed in low SOM soils. The random forest model showed that LOC had the largest amount for fungal interactions in low SOM soils (10%), followed by recalcitrant organic carbon (ROC). In comparison, LOC contributed less to the variations in fungal interactions in high SOM soils (7.4%). With globally increasing soil carbon loss, the limition of the carbon resources, especially the reduction of LOC, may reduce the stability and ecological functions of soil fungal communities.

Responses of Labile Organic Nitrogen Fractions and Enzyme Activities in eroded Mollisols After 8-year Manure Amendment.

Soil erosion will cause a degradation in soil nitrogen supplying capacity (SNSC) and manure amendment is an effective way to restored eroded soils. Both labile fractions of soil organic N (SON) and N transformation enzymes are indicators for SNSC, but the effect of manure amendments on labile SON fractions and the relationship between labile SON fractions and enzyme activities remains unclear. In this study, five degrees of erosion were simulated in Mollisols (removal of 0, 5, 10, 20 and 30 cm of topsoil) to analyse the changes in labile SON fractions and nitrogen transformation enzyme activities after 8-year manure amendment. We found that soil total N (TN), labile SON fractions and enzyme activities all increased after manure amendments. The largest labile SON fraction was particle organic nitrogen (POM-N) and the second was light fraction organic nitrogen (LFOM-N), which accounted >60% for TN in total. Correlation analysis showed that both urease and protease activities were significantly correlated with POM-N, LFOM-N, microbial biomass N and dissolvable organic N, indicating that both urease and protease activities can be used to predict labile SON pools and enzyme activities worked similarly in indicating SNSC with labile SON fractions. Altogether, 8-year manure amendment could recover SNSC of lightly eroded Mollisols to natural levels, i.e. erosion depths at 5 cm and 10 cm; however, it is not able to recover SNSC in Mollisols suffering severe erosion.

[Effect of temperature, rainfall and soil properties on farmland soil nitrification].

Climate conditions, soil properties and management practices control soil nitrification process which affects nitrogen cycling and balance in agro-ecosystems. The interaction of temperature, rainfall, soil type and fertilization on the soil nitrification process was studied by a soil transplantation experiment installed in 3 experiment stations of Chinese Ecological Research Network, i.e., Hailun, Fenqiu and Yingtan Agroecological Experiment Station, which represents middle temperature, warm temperature and middle subtropical zone, respectively. Three types of cropland soils were selected, i.e., neutral black soil (Phaeozem), alkaline Chao soil (Cambisol) and acidic red soil (Acrisol). Then one-meter depth soil profiles for each soil were transplanted in 3 stations to build the field experiment. The two-year experimental results (2006-2007) showed soil nitrification intensity (SNI) changed with the temperature and rainfall during the maize tasseling stage. From Hailun to Yingtan, with an increase of monthly average temperature from 22.3 degrees C to 26.8 degrees C and the monthly rainfall from 100.8 mm to 199.6 mm, SNI decreased by 64.2%-67.2% for black soil, 52.1%-52.5% for Chao soil, and 41.7%-75.2% for red soil, respectively. There were significant negative correlations between SNI and temperature and rainfall, with a correlation coefficient of r = -0.354 (p < 0.01) and r = -0.290 (p < 0.01), respectively. The total number of soil nitrobacteria and the intensity of soil nitrification was affected by soil types, which increased in a sequence of Chao soil > black soil > red soil. Among soil properties, pH affected SNI significantly, with a correlation coefficient of r = 0.551 (p < 0.01). In generally, climate condition (temperature and rainfall), soil type and fertilization present an integrated impact on soil nitrification process, and there were significant interactions of climate x soil type, climate x fertilization, soil type x fertilization, and climate x soil type x fertilization.